Absorbed in a Quasar Spectrum

Many people seem to think that astronomers spend all their time looking at pretty pictures of stars and galaxies. Actually a large part of observational astronomy isn’t about making images of things but doing spectroscopy. In fact the rise of astronomical spectroscopy is what turned astronomy into astrophysics. But that’s not to say that spectra can’t be pretty either. Here is an example (from here) which shows the light from the quasar HE0940-1050 taken by the UVES instrument mounted on ESO’s Very Large Telescope in Chile.

This quasar is an interesting object, at a redshift of z= 3.0932 (which converts to a look-back time of about 11.6 billion years). The dark bands and lines you can see in the spectrum are caused by absorption of the light from the quasar by clouds of hydrogen gas between the quasar and the observer; the strength of the absorption indicates how much gas the light from the quasar has travelled through. The absorption occurs at a particular wavelength corresponding to the Lyman-α transition but, because the clouds are all at different redshifts, each produces a line at a different observed wavelength in the quasar spectrum. There are many lines, which is why the collection of clouds responsible for them is often called the Lyman-α Forest. In effect the quasar sample is very much like a core sample, as if we were able to drill back in time to the quasar through the material that lies along the line of sight.

This spectrum is particularly remarkable because of the number of faint lines that can be seen: it’s like a detailed DNA Fingerprint of cosmic structure. It’s also very pretty.

4 Responses to “Absorbed in a Quasar Spectrum”

The figure you show is pretty, but it may be worth mentioning that it was created by the ESO outreach people (I asked the first author of the paper) from a so-called 1D spectrum (flux density vs wavelength), probably one where the shape of the continuum of the quasar has been taken out. It is many years since I’ve seen a 1D spectrum in an ESO press release.

“This quasar is an interesting object, at a redshift of z= 3.0932 (which converts to a distance of about 11.6 billion light years). “

That’s the approximate light-travel time in the concordance model, but not what most people would think of as a distance. At such redshifts, distances can vary appreciably depending on definition. For this redshift, typical distances used in cosmology range from about 5 to about 90 billion light years. In such cases, quoting any distance is often more trouble than it’s worth: those who are familiar with the topic can calculate what they want, those not are misled if one mentions just one and confused if one mentions more than one.